Optically programmable arbitrary temporal profile electric generator

ABSTRACT

The present invention relates to an optically programmable electric generator of arbitrary time profiles, which comprises a first ultrahigh frequency triggering line ( 10 ) and a second ultrahigh frequency discharge line ( 12 ) resistively coupled, by points, the first line being triggered by a voltage transition of duration less than one nanosecond, at least one point being taken off from the first line ( 10 ) by at least one photoconductor in variable resistance mode, directly coupled to the second line, illuminated by a programmable light source ( 13 ). A resistive load is connected at the output (S) of this generator.

TECHNICAL FIELD

The present invention relates to an optically programmable electricgenerator of arbitrary time profiles.

PRIOR ART

The documents “The National Ignition Facility Front End Laser System” byS. C. Burkhart, et al., Proceeding of the First Annual InternationalConference on Solid State Lasers for Application to Inertial ConfinementFusion Conference, pp. 48-58, SPIE Vol. 2623, April 1995; “TemporalPulse Shaping of Fiber Optic Laser Beams” by S. C. Burkhart, et al.,Inertial Confinement Fusion, pp 75-81, ICF Quarterly Report, Vol. 6, No.2, February 1996; and “Driver/Source/Time Shaping” by Y. Hourmand,Technical note DRIF/DCRE 443/96 dated Jul. 2, 1996, describe apulse-forming device controlled by a computer using GaAs field-effecttransistors, but these devices are limited frequency-wise andvoltage-wise.

DESCRIPTION OF THE INVENTION

The present invention relates to an optically programmable electricgenerator of arbitrary time profiles, characterised in that it comprisesa first ultrahigh frequency triggering line and a second ultrahighfrequency discharge line resistively coupled, by points, in that thefirst line is triggered by a voltage transition of duration less thanone nanosecond, for example a pulse or a step, in that at least onepoint is taken off from the first line by at least one photoconductor instatic resistance mode, directly coupled to the second line, illuminatedby a programmable light source, a resistive load being connected at theoutput of this generator.

Advantageously a capacitor can optionally be disposed between eachphotoconductor and the second line making it possible to provide D.C.isolation of the first line from the second. The photoconductors aremade of semiconductor materials taken from among the followingmaterials: silicon, gallium arsenide. Each programmable light source canbe an ultraviolet/visible/infrared gas tube, or a laser or lightemitting semiconductor diode.

Advantageously each light source can be supplied by an independentcurrent source controlled voltage-wise, the input signal of this currentsource being delivered by a programmable multi-way voltage generatordriven by a computer.

Advantageously each programmable light source produces a variableillumination, programmed with reference to a set point, which definesthe value of the resistance of the photoconductor, to which it iscoupled, at the instant the generator is triggered.

Advantageously the two lines are straight strip lines made from metallicdeposits on two substrates respectively associated with two dielectricsof different dielectric constants. The two ultrahigh frequency lineseach have a characteristic resistance of between 10 Ω and 100 Ω, thesubstrate having a loss tangent tg δ≈10⁻³ at 10 GHz. The triggering ofthe first line can be an electrical or photoelectrical triggering, ofindexed or pulsed type. A filter can be disposed between the output andthe load, which makes it possible to smooth the time profile obtained.

Advantageously the programming step is between thirty picoseconds andone nanosecond. It is defined by the spacing between twophotoresistances.

Such an electric generator of time profiles, preprogrammed byphotoconductors, which is triggered by another generator, allows thetime shaping (TS) of short pulses. Depending on the size of the timeslot in which the profile must be programmed, and the triggeringgenerator available, the step Δt can vary between a few tens and a fewhundreds of picoseconds.

In an advantageous use, this generator can be coupled to anelectro-optical crystal using the Pockels effect to implement a laserpulse amplitude modulator.

Compared with the devices of the prior art, the generator of theinvention has many original characteristics, and notably the following:

The coupling by photoconductors in optically preprogrammed resistancemode, and therefore the time shaping generator function, is programmableelectronically.

The time shaping principle is based on an electrical circuit forprogramming by components preprogrammed in “quasi-static” mode, asopposed, for example, to the devices described in (1) “National IgnitionFacility Front End Laser System” by S. C. Burkhart, R. J. Beach, J. H.Crane, J. M. Davin, M. D. Perny, and R. B. Wilcox, Solid State Lasersfor Application to Inertial Confinement Fusion Conference, proceedings,page 48, Monterey, Calif., May 31-Jun. 2, 1995; (2) “Temporal PulseShaping of Fiber Optic Laser Beams” by S. C. Burkhart and F. A. Penko,Inertial Confinement Fusion, ICF Quarterly Report, Jan. -Mar. 1996, Vol.6, No. 2; and (3) “Driver/Source/Time Shaping” by Y. Hourmand, Technicalnote DRIF/DCRE 443/96 dated Jul. 2, 1996, using components switchedunder dynamic conditions.

There are no high-speed active components, apart from triggering, of theGaAs field-effect transistor switch type, as in (1) “National IgnitionFacility Front End Laser System” by S. C. Burkhart, R. J. Beach, J. H.Crane, J. M. Davin, M. D. Perny, and R. B. Wilcox, Solid State Lasersfor Application to Inertial Confinement Fusion Conference, proceedings,page 48, Monterey, Calif., May 31-Jun. 2, 1995; (2) “Temporal PulseShaping of Fiber Optic Laser Beams” by S. C. Burkhart and F. A. Penko,Inertial Confinement Fusion, ICF Quarterly Report, Jan. -Mar. 1996, Vol.6, No. 2; and (3) “Driver/Source/Time Shaping” by Y. Hourmand, Technicalnote DRIF/DCRE 443/96 dated Jul. 2, 1996, for example. In such a case,the high-frequency behaviour is limited by the speed of the field-effecttransistors.

In the generator of the invention, the high-frequency limitation is duesolely to the triggering generator and the losses in the lines.

It is possible to obtain a very large dynamic range in the programmingof the photoconductors Rj in resistance mode, the photoconductors beingilluminated in quasi-continuous mode. The limit on the dynamic range isdue to the “signal/noise” ratio on the preprogrammed value Rj, andtherefore the dark current of the photoconductor and the noise of theillumination source. This ratio, which can exceed 30 dB, is a functionof the photoconductor implementation conditions.

There is no jitter of the time steps with respect to the triggeringtransition, by comparison with devices not using a single triggeringsource.

The invention is simple and versatile from an electronic andoptoelectronic point of view. It is of low cost, if the laser diodeoptical programming sources (at present still expensive) are replaced bygas tubes. It has a small volume with regard to integrability, comparedwith solutions using discrete electrical components, for ultrahighfrequencies, of the variable attenuator type.

The use of photoconductors under quasi-static conditions, for making thetime shaping generator programmable, makes it possible to combine allthe advantages for a better “high voltage/large dynamic range/highfrequency” compromise. With triggering using a photoconductor, theshaping of electrical or optical pulses (via an electro-opticalmodulator) can be performed with the following characteristics:

time step between 30 and 100 picoseconds;

voltage levels exceeding 5 kVolts;

programming dynamic range between 10 and 30 dB minimum.

One of the immediate applications of the generator of the invention isthat of allowing the time shaping of an optical pulse provided by alaser, by chopping by means of an electro-optical modulator according toa predefined profile at the instant of the arrival of the triggeringpulse.

A large number of other applications can be envisaged, as soon as it isnecessary to provide, at a resistive load, a voltage profile fallingwithin a previously defined time template, and within a range of levelswhich can extend from a few volts to a number of kilovolts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an embodiment of the electric generator according tothe invention;

FIG. 2 illustrates an example circuit for the design of the electricgenerator according to the invention;

FIGS. 3 and 4 illustrate two signal shapes obtained at the output of theelectric generator according to the invention; FIG. 3 depictingtriggering by a step and FIG. 4, triggering by a short pulse.

DETAILED DESCRIPTION OF EMBODIMENTS

The programmable generator of the invention is based on an assembly oftwo resistively coupled ultrahigh frequency lines 10, 12, incorporatingphotoconductors 11 programmable electronically by laser diodes 13 (orother gas and solid optical sources) and triggered by a high-speedswitch placed at P which provides a very short voltage transition,typically of a few tens to a few hundreds of picoseconds. This switchmakes it possible to trigger the generator, by electrical orelectro-optical means (a photoconductor type switch).

As illustrated in FIG. 1, this generator comprises a first line 10 (thetriggering line) triggered by means of a pulse (Dirac) or a shorttransition (step) of voltage (P being the triggering point), whichallows propagation of this signal at constant speed.

A number of points, equidistributed or not, are defined on this firstline 10, and coupled to as many photoconductors in resistance mode 11,directly or through a capacitor, on a second line 12. The function ofthese capacitors is to provide DC isolation of the first line from thesecond, which makes it possible to limit the consumption of thegenerator. The choice of using, or not, a connecting capacitor is alsorelated to possible constraints on the dimensioning of thephotoconductors 11 depending on their bias state. The second line 12(the discharge line) is coupled to the first only by means of thesephotoconductors. The distributed stray couplings are limited as much aspossible by controlling the geometry, the dimensions and thedistribution of the earths of the assembly. The coupling photoconductors11 are programmable, which makes it possible to obtain a generator ofelectrical profiles programmable voltage-wise. The photoconductors invariable resistance mode are made based on semiconductor materials ofsilicon (Si), gallium arsenide (GaAs) or some other type, illuminated byan assembly of programmable light sources which can be:

an assembly of “ultraviolet/visible/infrared” gas tubes;

an assembly of light emitting semiconductor diodes (LEDs) or laserdiodes (LDs).

FIG. 1 depicts laser diodes 13 which are supplied by independent currentsources 14 controlled voltage-wise. The input signals V1(t1), Vn(tn) ofthese current sources are delivered by a programmable multi-way controlgenerator 15 via a control bus 16, this generator being driven by asignal E originating from a computer.

Each of the light sources 13 produces a variable illumination,programmed with reference to a set point, which defines the value of theresistance of the photoconductor to which it is coupled at the instantthe generator is triggered.

The lines 10 and 12 are “strip” type lines made from metallic depositson dielectric substrates 22, 23. These substrates 22, 23 have lowlosses, in order to be able to pass 3 to 10 GHz, and have a loss tangenttgδ ≈10⁻³ at these frequencies. The basic parameters of each of the twolines 10 and 12 are the characteristic impedance and the distanceseparating two neighbouring photoconductors 11, and therefore the timestep.

In the case of a high-voltage configuration, the nature and thickness ofthe dielectric are chosen in order to meet the voltage performancecriteria.

The two lines 10 and 12 do not operate with the same time step and thedifference in the two steps is exploited to define the shaping.

In operation, the first line 10 propagates the triggering transition,which defines an elementary time step between photoconductors, andinduces a current transition ΔI=ΔV/R, R being the value of resistance,this transition being propagated and back-propagated in the second line12.

As illustrated in FIG. 1, two straight strip lines 10 and 12 are used,associated with two dielectrics 20 and 21 of different permittivities(∈). A ratio of 4 in the values of ∈ makes it possible to obtain a ratioof 2 in the propagation speeds, that is a ratio of 2 in the time stepsif the photoconductors are set out as illustrated in the figure. Thissolution is simple from a technological point of view. It suffices tocontrol the characteristic resistances by means of the widths of the twolinear strips. These characteristic resistances can vary typically, forconventional dielectrics, from a few ohms to 200 ohms.

In the generator of the invention, each line has a characteristicresistance of between 10 Ω and 100 Ω, these two lines being able to haveidentical or different characteristic resistances. The choice of thesecharacteristic resistances has an influence on the generatoroptimization criteria: electrical efficiency, high-voltage performance,and triggering source characteristics.

In this embodiment, the time step is between thirty picoseconds and onenanosecond.

The photoconductors (Rj) are very slightly capacitive, which allows thegenerator to operate at high frequencies. For a time step equal to 100psec on the triggering line and a range of values Rj=1 to 200 ohms forexample, the capacitance associated with Rj must not exceed Cj=0.2 pF. Aphotoconductor geometry in the form of a long bar is therefore used,uniformly illuminated over its length (Dj) and in its volume. For Dj=1cm, a width of 1 to 3 mm, and an Si or GaAs type photoconductormaterial, Cj does not exceed a few tens of fF.

Under steady state conditions there are no high frequencies, as far asthe constraints due to the photoresistances are concerned.

If the case of high-voltage triggering by photoconductor, coupled to anenergy storage line, is considered, a transition of 5 kV with a risetime less than 100 psec can thus be applied at the input of thetriggering line 10.

The generator of the invention is preprogrammed at the instant oftriggering, i.e. when the triggering transition is sent on the firstline 10, at P.

There can be electrical or photoelectrical triggering of the first line10: in the latter case, a photoconductor illuminated by a laser is used.Different types of indexed or pulsed triggering can be used.

The output (S) of the generator of the invention is connected to aresistive load: this can be an electric generator operating on aresistive load or an electro-optical modulator.

A filtering element can be placed between the output and the load, whichmakes it possible to smooth the time profile obtained. Smoothing canalso be obtained by increasing the value of the rise time of thetransition at P.

FIG. 2 illustrates an example circuit for the design of the generatoraccording to the invention. There is a four-point operation based onphotoconductors preadjusted in quasi-static mode, with a triggering line25 with a constant 10 ohm impedance, a discharge line 26 with a constant50 ohm impedance, a circuit 27 for triggering by GaAs serialphotoconductor, RC networks 28 representing the equivalent circuit ofthe quasi-continuous photoconductor (QCW) and an electro-opticalmodulator 29 (with, in this precise case, compensation for themechanical effects in the modulator).

The line elements “Ttrigger” and “Tdischg” represent the sections ofstrip line between which the resistive coupling points are installed.

These couplings are carried out with no capacitive connection. Theelements “R_phot”, “C_phot” and “L_phot” represent respectively aprogrammed photoresistance, and an associated stray capacitance andself-inductance, these stray elements being minimized as much aspossible. The switches “S_trigger” and “S_turnoff”, controlled by thevoltage generators “V_trigger” and “V_Tturnoff”, are a simplifiedrepresentation of the injection generator, and of the turn-off switchwhich determines the shaping time slot. “S_trigger” can in particular bea photoconductor to which is coupled a laser providing short pulses (ofthe order of 10 to 30 picoseconds). The capacitor “Cstorage” isnecessary from an electronic point of view for storing the energy beforethe instant of triggering (“serial” type triggering).

This example embodiment assumes the generator coupled to a travellingwave electro-optical modulator represented simply by a load resistor“R_load”. The voltage source “V_turnoff”, which is non-zero, has thefunction of compensating for the mechanical (and other) effects inducedin the modulator crystal by the highvoltage transitions. Thus, it ispossible to control the contrast when the system is turned off, afterthe rear transition of the time slot “T”. The simulations are performedin the two extreme cases where the following are used as the triggeringsignal:

a step, with a photoconductor for which the life of the charge carriersis large compared with “T” as illustrated in FIG. 3;

a short Dirac type pulse, with a photoconductor for which this same lifeis short or of the same order of magnitude as T, as illustrated in FIG.4.

What is claimed is:
 1. An optically programmable electric generator of arbitrary time profiles, characterised in that it comprises a first ultrahigh frequency triggering line (10) and a second ultrahigh frequency discharge line (12) resistively coupled, by points; in that the first line is triggered by a voltage transition of duration less than one nanosecond; in that at least one point is taken off from the first line by at least one photoconductor (11) in variable resistance mode, directly coupled to the second line, illuminated by a programmable light source (13), a resistive load being connected at the output (S1, S2) of this generator.
 2. A generator according to claim 1, in which a capacitor is disposed between each photoconductor (11) and the second line (12) making it possible to provide D.C. isolation of the first line (10) from the second (12).
 3. A generator according to claim 1, in which the photoconductors (11) are made of semiconductor materials taken from among the following materials: silicon, gallium arsenide.
 4. A generator according to claim 1, in which each programmable light source (13) is a gas tube.
 5. A generator according to claim 1, in which each programmable light source (13) is a laser or light emitting semiconductor diode.
 6. A generator according to claim 5, in which each programmable light source (13) is supplied by an independent current source (14) controlled voltage B wise, the input signal (V1(t1), Vn(tn)) of this current source being delivered by a programmable multi-way voltage generator (15) driven by a computer.
 7. A generator according to claim 1, in which each programmable light source produces a variable illumination, programmed with reference to a set point, which defines the value of the resistance of the photoconductor, to which it is coupled, at the instant the generator is triggered.
 8. A generator according to claim 1, in which the ultrahigh frequency lines (10, 12) are straight strip type lines made from metallic deposits on two substrates (22, 23) of different dielectric constants (∈). 